Stop libdispatch bulk after value capture failure

[fallintoplace]

Summary

• stop libdispatch bulk after predecessor value capture fails
• add regression coverage for a throwing predecessor value

The receiver already sends set_error when value capture fails. The old code continued into the completion path after that error.

Tests:

• cmake --build build --target test.exec -j 8
• ./build/test/exec/test.exec --reporter compact

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[ericniebler]

/ok to test 3fefbde

[fallintoplace]

Hey Eric, thanks for approving. I found one more edge case in libdispatch bulk capture for set_value(T&). I’m going to push a small follow-up with the forwarding fix and a regression test. The existing tests pass with it.

[ericniebler]

/ok to test 49b48e9